This invention relates to prosthetic implants for joint structures and particularly to a prosthetic implant structure and assembly method for arthroplasty involving ball and socket type joints.
Arthroplasty or joint replacement for ball and socket type joints includes providing an implant to replace the natural anatomic ball or head of the joint. Prosthetic implants for ball and socket type joints are classified as either unipolar or bipolar. A unipolar implant includes simply a head adapted to replace the ball structure of the natural joint and to articulate with the anatomic socket structure. Bipolar implants provide for articulation within the implant itself in addition to articulation with the anatomic socket structure. The added articulation within the implant structure reduces wear on the natural socket structure and provides better range of motion and freedom of movement in the joint.
Bipolar implants for ball and socket type joint structures such as the hip joint, for example, include a head or ball, a shell adapted to be received in the anatomic socket structure, and a receptacle in the shell. An insert member is commonly received in the shell receptacle and the implant head is adapted to be received directly in the insert member. In the case of a hip joint bipolar prosthesis, the implant head is adapted to be attached to the top of the femur after the natural femur head is resected. The implant head articulates with the insert in the shell and the shell articulates with the anatomic socket or acetabulum.
The manner in which the several components of a bipolar prosthetic implant are assembled is critical to the functioning of the implant. The implant head should be easily removable from the insert member and shell yet must be retained in the shell securely with minimal play so that the implant head freely articulates in the insert. To enhance the flexibility of the surgical implantation procedure, the implant should also be adapted for relatively quick assembly and disassembly with a minimum of small parts.
Prior prosthetic implants for ball and socket type joints fail to meet one or more of these requirements. For example, U.S. Pat. No. 3,863,473 to AVERILL discloses a prosthetic hip joint implant that uses a tab formed on the insert member and a cooperating groove on the inner surface of the shell to retain the insert member in the shell. This tab arrangement, however, requires that the tab deflect inwardly as the insert member is placed in the shell. When the tab snaps outwardly in the groove formed in the shell, the arm on which the tab is formed also moves outwardly and leaves excessive play between the head and the insert member.
The implant structure shown in U.S. Pat. No. 4,718,918 to KENNA retains the insert member in the shell with a lip formed on the inside surface of the shell and a cooperating groove formed on the outside surface of the insert. This locking or retaining arrangement results in either play between the insert member and the head received therein or play between the insert member and the shell. The retaining arrangement also makes it difficult to remove the insert member once it is connected with the shell.
Another type of prosthetic hip joint implant is shown in U.S. Pat. No. 4,241,463 to KHOVAYLO. This patent is directed to a prosthetic implant that retains the insert member in the shell with a rigid lip on the inside of the shell and an groove formed in the insert similarly to the KENNA patent. However, KHOVAYLO uses a retaining slip ring within the insert member itself to retain the head within the insert member. The space required for the ring in the insert member greatly reduces the bearing surface available for the head, and results in some degree of play between the insert member and the head.